Deep wells such as oil and gas wells are typically drilled by rotary drilling methods. Some such methods are described in Walter U.S. Pat. No. 4,979,577. Apparatus for rotary drilling typically comprises a suitably-constructed derrick. A drill string having a drill bit at its lower end is gripped and turned by a kelly on a rotary table or by a top drive.
During the course of drilling operations, drilling fluid, often called drilling mud, is pumped downwardly through the drill string. Drilling fluid exits the drill string at the drill bit and flows upwardly along the well bore to the surface. Drilling fluid caries away cuttings, such as rock chips.
The drill string is typically suspended from a block and hook arrangement on the derrick or from the top drive. The drill string comprises drill pipe, section of drill collars, and may comprise drilling tools such as reamers, drilling jars and shock tools. The drill bit is located at the extreme bottom end of the drill string.
Drilling a deep well is an extremely expensive operation. Great cost saving can be achieved if drilling can be made more rapid. A large number of factors affect the penetration rates.
The weight on the drill bit has a very significant effect on drilling penetration rates. If rock chips are adequately cleaned from the rock face at the bottom of the well hole, doubling of the weight on bit (WOB) will roughly double the penetration rate. It has been established that when the drilling fluid exits the drill bit in jets, better cleaning of the rock face is achieved. This is better explained in (Walter) U.S. Pat. No. 4,979,577. Further information on rotary drilling and penetration rates may be found in standard texts on the subject, such as Preston L. Moore's Drilling Practices Manual, published by Penn Well Publishing Co. (Tulsa, Okla.).
In an effort to increase penetration rates a number of down hole devices which exploit the water hammer effect to create pulsation of the flow of the drilling fluid have been developed. Such devices are useful in improving hydraulic cleaning of the bit and rock face. These devices are commercially used in combination with shock tools. Examples of such drilling fluid pulsing devices can be found in U.S. Pat. No. 4,819,745 (Walter), U.S. Pat. No. 4,830,122 (Walter), U.S. Pat. No. 4,979,577 (Walter), U.S. Pat. No. 5,009,272 (Walter), U.S. Pat. No. 5,190,114 (Walter).
In a typical shock tool a pressure pulse can act on a piston. This results in a force having a magnitude related to the area of the equalization piston multiplied by the amplitude of the pressure pulse. Since the area of the shock tool piston is relatively small the resulting force is beneficial but is often not significant.
There is a need for drilling methods that are more cost-effective than currently-used methods. There is a need for apparatus useful in the implementation of such methods.